The present invention relates to an ion trap mass spectrometer which performs mass analysis of trapped ions generated in external ion source and injected into an ion trap.
An ion trap mass spectrometer comprises a ring electrode and two end-cap electrodes arranged opposite in direction to each other sandwiching the ring electrode, as shown in FIG. 2. A direct current voltage U and a radio-frequency voltage V.multidot.cos .OMEGA.t are applied between the electrodes to form a quadrupole electric field in the electrode space. Stability of path of ions trapped in the electric field is determined by a dimension of the instrument (inner diameter r.sub.0 of the ring electrode), the direct current voltage U, the amplitude V and the angular frequency .OMEGA. of the radio-frequency voltage applied to the electrodes, and values a and q given by a mass-to-charge ratio of an ion (Equation (1)). EQU a={8eU/r.sub.0.sup.2 .OMEGA..sup.2 }(Z/m), q={4eV/r.sub.0.sup.2 .OMEGA..sup.2 }(Z/m) (1)
where Z is an ionic charge number, m is a mass of the ion, e is the elementary electric charge.
FIG. 3 shows a stability diagram expressed by the range of the values a and b giving stable trajectories in the ion trap volume (quadrupole electric field space). In general, since only the radio-frequency voltage V.multidot.cos .OMEGA.t (RF trap voltage) is applied to the ring electrode, all ions whose values a and q are on the straight line a=0 inside the stability region are stably oscillated and trapped inside the ion trap volume. At that time, the ions are different in (0, q) point on the stability region (FIG. 3) depending on the value of mass-to-charge ratio (m/Z), and sequentially aligned on the a-axis in the range of q=0.about.0.908 determined by Equation (1) from ions having a large value of mass-to-charge ratio to a small value of mass-to-charge ratio. Further, the oscillation characteristics of ion oscillations in the ion trap volume are different depending on the (a, q) point on the stability region (FIG. 3).
One type of the ion trap mass spectrometer is operated with a resonance ejection mode, in which by using the phenomenon that ions oscillate with different frequency depending on a mass-to-charge ratio m/Z, an auxiliary alternating current electric field having a specific frequency is generated in the ion trap volume, and only the oscillation of ions resonating with the auxiliary alternating current electric field is amplified to mass separate the ion species by ejecting from the ion trap volume.
There are various types other than the above. In these types of the ion trap mass spectrometers, a mass distribution of ions composing a specimen can be measured by detecting the mass-to-charge ratio m/Z of ions which are mass separated and ejected.
There are two kinds of means for trapping ions of a specimen to be mass analyzed in an ion trap volume. That is, as described in Japanese Patent Application Laid-Open No.62-37861 and Japanese Patent Application Laid-Open No.2-103856, one method is that a neutral specimen to be mass analyzed is injected into an ion trap volume, and ionized in the ion trap volume by electron collision or the like, and then directly trapped in the ion trap volume; and the other method is that ions generated in an ion source external of an ion trap are injected in an ion trap volume to be trapped. During an ion injecting period (trapping period=storing-up period) (A) shown in FIG. 4, ion generation and stable trapping of ions are performed in a case of the former type, and ion injection and stable trapping of ions generated in the external ion source is performed in a case of the latter type. That is, even in the case where ions are generated in the external and the generated ions are injected into the ion trap volume, an RF trap voltage having a constant amplitude is generally applied during the ion injection period, as shown in FIG. 4.